This invention is directed in general to tone detectors and more specifically, to tone detectors which must function to detect tones on a communication channel in which speech or other signals may be present. This invention is specifically, but not exclusively, suited for dual tone multifrequency (DTMF) digit detection such as in a telecommunication system.
It is becoming increasingly important to be able to detect tones such as DTMF digits on a communication channel which may also carry other signals such as voice. An idealized detector detects a predetermined set of tones and does not mistake other signals such as speech as one of the predetermined tones. Services which are selectable by DTMF signaling over a telecommunication system, are becoming increasingly popular. However, since other background noise or signals may be present on the same channel which carries the DTMF digits, the tone detector or receiver, such as at a central office, must be capable of reliably detecting the DTMF digits while simultaneously minimizing false detections which may be caused by other signals or voice on the channel.
DTMF digit detectors must operate under difficult conditions. First, the detector must be able to correctly detect a received DTMF digit over a range of characteristics defined by signaling standards that define a valid digit, including variations of the frequency of the tone, amplitude of the tone (both as an absolute value and relative to the amplitude of the dual tone pair), and subject to intermodulation distortions and other noise within defined limits. Second, the DTMF detector must also minimize the number of false or talk-off detections due to the presence of speech. Many previous DTMF detectors have achieved reasonably good performance in one of these two requirements but has typically not been totally successful in simultaneously achieving both requirements. Thus, there exists a need for an improved tone detector which not only accommodates required variations of the tones but also minimizes false detection such as due to talk-off.